The present invention relates to a high temperature superconducting composite conductor and to a process for the manufacture of such a high temperature superconducting composite conductor.
The discovery of high temperature superconductive materials in the late 1980""s was soon followed by a desire to form such materials into tapes or similar shapes. Ideally such tapes should be physically strong, flexible, highly conductive and able to withstand strong magnetic fields without loss of current carrying capacity.
Processes generally referred to as xe2x80x9coxide powder in a tubexe2x80x9d (OPIT) have been developed. For example, a general process of fabricating superconductive tape involves initially preparing a superconductive powder, filling a tube or pipe of silver with the superconductive powder, sealing the pipe or tube, subjecting the pipe or tube to reducing or deforming operations to form tape, and finally sintering the reduced tape at high temperatures. Such reducing or deforming operations have typically included both a drawing stage and a rolling stage.
OPIT processes have several problem areas hindering their industrial application. First, silver is a major cost in the production of bismuth-based superconducting tapes for commercial applications. Current methods for reducing costs of bismuth-based conductors are limited due to a need for a wire drawing step in the production of the conducting tapes. Wire drawing results in a typical fill factor for the superconductor within the tapes to a limit of only about 30 percent by volume. This limitation results from the inability of the powder core to supply enough support during wire drawing due to lack of tensile strength. Hence, a significant amount of silver is needed for the wire to survive the drawing process.
Another drawback is the uniformity of the filaments after wire drawing and rolling. Rolling is nominally a plane-strain process. However, the process of rolling a round wire leads to a non-uniform distribution of stresses and strains within the wire as it is flattened and elongated. Hence, in a multifilamentary product, non-uniformity of the different filaments is a common problem. The outside filaments are often more porous and less uniform than interior filaments. Often, there are also many filament interconnections within the tapes. By xe2x80x9cinterconnectionsxe2x80x9d is meant that during the rolling process, the deformation of the silver/superconducting powder composite leads to two or more filaments combining or touching. This is a problem as these filament interconnects tend to have more secondary phases and porosity which leads to breakdown in the connectivity of the superconductor and to lower critical currents.
Another known process of forming high temperature superconductor tapes is referred to as continuous tube forming and filling. Such a process involves taking a silver strip and then placing superconductive precursor powder onto the silver strip as it is continuously formed into a tube in a process like the welding industry has used to make tubular welding wire. However, the reported fill factor on this process is only about 25 percent.
As a result of persistant studies, the inventors have found a process to overcome these drawbacks and to produce a product without these limitations.
Accordingly, it is an object of the present invention to provide a high temperature superconducting composite conductor having a low silver content and a high fill factor of superconductive material.
It is another object of the present invention to provide a high temperature multifilamentary superconducting composite conductor wherein the multiple filaments are substantially uniform in shape and/or dimensions.
It is a further object of the invention to provide a process of preparing a high temperature superconducting composite conductor having a low silver content and a high fill factor of superconductive material.
It is still another object of the invention to provide a process of preparing a high temperature multifilamentary superconducting composite conductor wherein the multiple filaments are substantially uniform in shape and/or dimensions.
Still another object of the invention is to achieve plane-strain deformation to obtain optimally densified cores.
To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, the present invention provides a process of preparing a high temperature superconducting composite conductor including filling rectangular-shaped tube of a noble metal with an oxide superconductor mixture; placing the filled rectangular-shaped noble metal tube into a secondary metal matrix containing a opening adapted for insertion of the filled rectangular-shaped noble metal tube; and, rolling the secondary metal matrix containing the filled rectangular-shaped noble metal tube to form a rolled product of a preselected thickness. The process can further include heating said rolled product at temperatures and for time sufficient to form a high temperature superconducting composite conductor.
The present invention further provides a process of preparing a high temperature superconducting composite conductor including filling rectangular-shaped tube of a noble metal with an oxide superconductor mixture; and, rolling said filled rectangular-shaped noble metal tube to form a rolled product of a preselected thickness. The process can further include heating said rolled product at temperatures and for time sufficient to form a high temperature superconducting composite conductor.
The present invention further provides a high temperature superconducting composite conductor including a high temperature superconducting material surrounded by a noble metal layer, the high temperature superconducting composite conductor characterized as having a fill factor of greater than about 40 percent. In one embodiment of the invention the conductor can be further characterized as containing multiple cores of high temperature superconducting material surrounded by a noble metal layer, said multiple cores characterized as having substantially uniform geometry in the cross-sectional dimensions.
The present invention still further provides a high temperature superconducting composite conductor prepared by the above process.